Research On Adsorption Of Fluoride From High-fluorine Groundwater By Nanoscale Iron Oxide Composites

Fluoride contamination in groundwater is of a global environmental problem.The distribution of high fluorine groundwater is extensive in China,especially in the vast northern regions in which people have chronicallyfaced the potential threat of endemic fluorosis caused by high fluorine groundwater.Therefore,effective prevention and control of fluoride contamination in groundwater is particularly urgent.Iron oxide is one of the most intensively studied adsorbent materias used for fluoride removal from groundwater due to it is readily available and has high affinity for ions such as fluoride,etc.In this study,ferroferric oxide,with relatively excellent adsorption properties was selected and nanoscale ferroferric oxide was prepared by coprecipitation and characterized.Modification ofnanoscale ferroferric oxide using hydrochloric acids and preparation of composite iron oxide was then conducted.Furthermore,these adsorbent materials was used to remove fluoride from simulated high fluorinegroundwater.The effects of various environmental factors on the defluoridation efficiency were investigated.The thermodynamics and kinetics of adsorption processes were studied.and the mechanism was discussed.The conclusions based on research results were formed as follows:The particles of nanoscale iron oxides before and after modification have an averagespherical diameter of about 20 nm.Better particledispersion was found foracid-modified iron oxides.Spherical particle in the composite of aluminum hydroxide & ferric oxide which is coated with a gel-type substance,has a diameter in the range of 10 nm and 30 nm.The structural properties of nanoscale iron oxides before and after modification did not change significantly,while the specific surface area of iron oxide composite decreased,pore volume and pore size increased significantly,which shows significantchanges in surface structure and properties compared to nanoscale ferroferric oxide.The main chemical composition is samefor nano scale ferroferric oxide befoer and after modification with high purity and low impurities.Elements of Al and Swere found in iron oxide composites in addition to the Fe and O,which are presumed as that amorphous aluminum-containing substance adheres to the surface of ferroferric oxide crystal while certain sulfate ionsare doped in the composite.Studies in defluorination using nanoscale ferroferric oxide-based absorbents show that acid-modified iron oxide and iron oxidecomposite have a wide effective p H range compared to nanoscale ferroferric oxide.For simulated groundwater with fluorideconcentration of 5 mg/L,the optimum dosage of nanoscale ferroferric oxide before and after modification is 10/L and the removal rates are 71.2% and 85%,respectively.When the amount of iron oxide composite is 2 g/L,fluorine removal rate exceeds 96%.When the amount is increased to 4 g/L,the fluorine is considered as completely removed.In the initial stage of the reaction,fluorine removal increases with reaction time.After 30 minutes,fluorine removal increases slowly,and the adsorption equilibrium was achieved after 150-180 minutes.Kineticstudy shows that adsorption process with nanoscale ferroferric oxide-based absorbents fits pseudo-second-order kinetic model,indicating that chemical reaction related to the surface properties of these adsorbents is the rate control step.Thermodynamics study show that the adsorption of fluoride ions by modified nanoscale ferroferric oxide has both monolayer adsorption and multi-layer adsorption while results from adsorption by rion oxide composite better fit Freundlich adsorption isotherm model,which indicates predominant multi-molecular layersadsorption process.Equilibrium adsorption capacities of fluoride ion by modified nano-scale ferroferric oxideandiron oxides were respectively 2.4624 mg/g and 44.8430 mg/g.In general,coexisting inorganic anions and humic acids in groundwater has little effect on the removal of fluorine,while coexisting cations have little effect on the removal of fluorine at lower concentration and relatively positive effect at higher concentration.The mechanism of fluoride removal by modified nanoscale ferroferric oxide is considered as that changes in crystal surface structure enhances particle surface activity,and the formation of active centers of ferrous and ferric ions enhances the adsorption capacity of iron oxides to fluorine.With respect toiron oxide composite,most of the fluoride ions reacts with aluminum oxide on the surface of nanoscale iron particles to form a stable complex in the form of O-Al-F bond;part of fluoride ions isadsorbed to the surface of the adsorbent by electrostatic attraction;There is a specific adsorptionduring theprocess.